In the developing nervous system, migrating cells and axons are guided to their targets by cues in the extracellular environment. The netrins are a family of phylogenetically-conserved guidance cues that can function as diffusible attractants and repellents for different classes of cells and axons.sup.1-10. Recent studies in vertebrates, insects and nematodes have implicated members of the DCC subfamily of the immunoglobulin (Ig) superfamily as receptors involved in migrations toward netrin sources .sup.6, 11-13. The mechanisms that direct migrations away from netrin sources (presumed repulsions) are less well understood. In Caenorhabditis elegans, loss of unc-5 (which encodes the transmembrane protein UNC-5.sup.14) function causes defects in these migrations.sup.15, 16, and ectopic expression of unc-5 in some neurons can redirect their axons away from a netrin source.sup.17. However, the relationship between UNC-5 and the netrins has not been defined. We disclose herein vertebrate homologues of the C. elegans UNC-5, which define a novel subfamily of the Ig superfamnily, and whose mRNAs show prominent expression in various classes of differentiating neurons and we disclose that these vertebrate UNC-5 homologues are vertebrate netrin-binding proteins.